In all fuel cells, particularly where the oxidant is supplied by air, inert gaseous molecules, particularly nitrogen, diffuse through the electrolyte and accumulate on the fuel side (anode) of the cell. The accumulation of inert residue ultimately blocks the hydrogen fuel from reaching the anode catalyst and the electrolyte, which ultimately leads to significant loss of cell performance. Exhausting or venting of effluent from the anode, which invariably will contain residual fuel, is undesirable in most instances because unreacted fuel can pose a safety hazard and will generally be perceived as polluting the atmosphere. Typical fuel cell power plants are designed for operation on hydrocarbon fuels (natural gas, methanol or gasoline) and usually utilize the anode exhaust as a source of fuel for a burner required in the apparatus that processes the fuel to make it a hydrogen-rich stream; all fuel in the anode exhaust is combusted, so that no unburned fuel will leave the power plant.
Although some space and military applications have utilized hydrogen-fueled fuel cell power plants, with no fuel processor, and either dead ended the anode (not allowing any effluent therefrom) or simply dumped the anode exhaust into the environment, there are other uses for hydrogen-fueled power plants, particularly in vehicles such as automobiles, which can neither tolerate the performance degradation which can be caused by buildup of inert residues on the anode side nor tolerate a fuel-containing exhaust. A dead-ended fuel stream causes trace level impurities to accumulate, which requires a fuel-exit purge into the ambient.